1. Field of the Invention
The field of the invention relates to synchronous detection of RF signals. In particular, the invention relates to synchronous detectors providing low phase jitter in the detected signal.
2. Description of Related Art
Known synchronous detectors use phase-locked loops to regenerate a replica of the carrier signal for use in synchronous detection. Phase modulation of the pilot carrier is an inherent part of a vestigial sideband signal. When a phase-locked loop is used to synchronously detect a vestigial sideband signal, the phase-locked loop will lock onto the inherent phase modulation of the pilot and produce replica of the carrier signal having inherent phase noise induced thereon. This phase noise will result in a distorted output at the output of the synchronous demodulator. To minimize this phase noise, known phase-locked loops use loop filters with a narrow bandwidth This limits the phase-locked loop's ability to track phase and maintain phase coherence with the carrier signal when sudden phase shifts are injected into the modulated signal, for example, undesired phase modulation in the local oscillator of the tuner due to microphonic effects.
Synchronous detector technology is an important technology for high definition television, among other uses. The transmission techniques for high definition television is a subject of recent controversy. Some proponents desire QAM while other proponents desire VSB. For example, 16-QAM, quadrature amplitude modulation, is a technique where a sequence of four bit data nibbles are split into two separated sequences of two bit symbols per sequence. The two separated sequences of symbols are fed into the two modulation input ports of a quadrature multiplex type modulator. QAM output signals are doubled sideband signals where the sidebands bear no particular phase relationship to each other due to the asymmetry between the two separate sequences of symbols used in the modulation process.
In contrast, for example, 4-VSB, vestigial sideband, is a technique where the same sequence of four bit nibbles is constituted as a single sequence of four bit symbols where the VSB symbol rate is equal to the sum of the symbol rates of the two separated sequences of symbols used in QAM.
The digital high definition television Grand Alliance, including AT&T, Zenith, General Instrument Corp., the Massachusetts Institute of Technology, Thomson Consumer Electronics, Philips Consumer Electronics and the David Sarnoff Research Center, has selected VSB over QAM as the transmission technology for high definition television.
The importance of high performance synchronous detection of VSB or QAM signals to high definition television is obvious. However, the present invention has application to any transmission technology where unwanted phase modulation in the transmitted signal induces phases errors in the detected signal.